1. Field of the invention
The present invention relates to a projection type display apparatus for enlarging and projecting picture images, generated by picture forming means comprising a cathode ray tube (CRT) and other devices, onto a screen by means of a projection optical system.
2. Description of Related Art
The constitution of a projection type display apparatus of a prior art will be described below with reference to FIG. 1 and FIG. 2. In these figures, numeral 1 denotes a CRT which is picture forming means, numeral 2 denotes a projection lens, numeral 6 denotes a coolant liquid, numeral 11 denotes a screen, numeral 12 denotes a spacer and numeral 13 denotes a packing.
The CRT 1 and the projection lens 2 are usually used in three sets for red, blue and green images, though only one set is shown in the drawing for the purpose of describing the operation of the projection type display apparatus of the prior art. A picture image generated on the CRT 1 is enlarged and projected onto the screen 11 by the projection lens 2 thereby to form an image thereon. The CRT 1 and the projection lens 2 are fixed on either side of a spacer 12 with the space surrounded thereby being filled with the coolant liquid 6. The coolant liquid 6 is sealed by means of the packing 13 to prevent it from leaking. The coolant liquid 6 has both cooling effect to suppress heat-up of the CRT 1 and optical coupling effect to prevent unnecessary reflection at the boundary between the glass member and the air layer. Thus the coolant liquid 6 plays an important role to obtain high brightness and high contrast.
Having the constitution and operation as described above, the projection type display apparatus of the prior art has such problems as described below.
As described before, a projection type display apparatus generally employs three projection units each having the CRT 1 comprising a cathode ray tube and other devices and the projection lens 2. FIG. 3 is a schematic drawing illustrative of the key portion of the projection type display apparatus, where the projection units UR, UG and UB for red, blue and green images are so arranged that the optical axes thereof cross each other at the center of the screen 11. The projection unit UG for green image emits light beams at right angles to the screen 11, and the projection units UR and UB for red and blue images emit light beams with a required convergence angle of .theta.1 to the screen 11.
In order for the image on the CRT 1 and the screen 11 to satisfy the correct relationship of image forming, it is necessary to incline the projection lens 2 and the CRT 1 at a correction angle .theta.2 which is determined by the following equation. In addition, relative distance between the CRT 1 and the projection lens 2 is changed to adjust the focusing on the periphery. EQU tan .theta.2=(n/m) tan .theta.1 (1)
where n denotes the refractive index of the coolant liquid and m denotes the magnification factor (screen size/CRT size) of the projection lens. The spacers 12 for red and blue images shown in FIG. 2 are fabricated with the CRT-mounting surface and the projection lens-mounting surface being inclined by the correction angle .theta.2 determined by the above equation.
Recently, demands have been increasing for projection type display apparatuses which are capable of changing the screen size in a wide range from 70 inches to around 300 inches in a single unit. However, because changing the screen size requires to change the magnification of the projection lens, it is necessary to change the correction angle .theta.2 between the CRT 1 and the projection lens 2 according to the screen size as shown in the equation (1). Because the spacer 12 is an integral body as shown in FIG. 2 in the conventional apparatus, however, it must be operated with fixed correction angle .theta.2. Consequently, correct image forming conditions cannot be maintained resulting in images out of focus in the periphery of the screen when the screen size is other than the design size.
There is a conventional apparatus devised to solve the above problem as shown in FIG. 4, wherein the holding member for mounting the CRT 1 and the projection lens 2 is separated into holding members 3 and 4 while sealing the coolant liquid only for the holding member 4 of the CRT 1, and light beam is emitted through a glass window 15. Although this construction has an effect of cooling the CRT, there is an air layer between the holding members 3 and 4 which inhibits optical coupling which has been another effect of the coolant liquid, resulting in the generation of a large amount of unnecessary reflecting light in the boundary with the air layer, thereby leading to significantly deteriorated contrast of the projected picture image.
To solve these problems, there has been proposed an apparatus having no air layer existing between the CRT 1 and the projection lens 2, which is capable of adjusting the relative angle and distance between these components.
FIG. 5 is a cross sectional drawing illustrative of the constitution of a projection unit composing a conventional projection type display apparatus, for example, disclosed in the Japanese Utility Model Application Publication H2-41979 (1990), and FIG. 6 shows the exploded perspective view thereof. Numeral 21 denotes a CRT or cathode ray tube, numeral 22 denotes a spacer block made of a material of good thermal conduction such as aluminum diecast. The spacer block 22 has a boss 22c having a threaded hole 22d on the side where the CRT 21 is mounted. Numeral 23 denotes a holding plate having a contact hole 23a which contacts with the external wall 21b of the CRT 21 and has a set hole 23b at each of the four corners. In order to press the holding plate 23 uniformly against the CRT 21, a post 24 with one end being in contact with each of the bosses 22c of the spacer lock after passing through the set hole 23b, and a spring 25 surrounding the post 24 with an end being in contact with the holding plate 23 are provided. The CRT 21 is clamped onto the spacer block 22 by the compressive force of the spring 25, by setting a screw 27 with a washer 26 through the post 24 and screwing it into the threaded hole 22d of the boss 22c. Numerals 36, 37, 38 denote a deflection yoke, a focus magnet and a CRT board, respectively mounted on the CRT 21.
On the other hand, a lens barrel 28a of the projection lens 28 is mounted by means of screws 35 on the side of the spacer block 22 opposite to the CRT. The space between the face 21c of the CRT 21 and the CRT mounting surface 22f of the spacer block 22, and the space between a lens surface 28b of the projection lens 28 and a lens mounting surface 22g of the spacer block 22 are provided with a CRT packing 29 and a lens packing 30, respectively, in order to keep the space 31 surrounded by the CRT 21, the projection lens 28 and the spacer block 22 liquid-tight.
Numeral 32 denotes a coolant liquid such as ethylene glycol which keeps the light emitted by the CRT 21 from returning to the fluorescent surface thereby to prevent the contrast of projected image from deteriorating and, at the same time, to transmit the heat generated on a face 21c of the CRT 21 during operation of the projector to the spacer block 22 thereby dissipating the heat to the outside. Numeral 33 denotes a tank which accommodates an excess of the coolant liquid 32 when it undergoes thermal expansion.
FIG. 7 is a cross sectional drawing illustrative of a projection unit composing a conventional projector, for example, disclosed in the Japanese Patent Application Laid-Open H4-352141 (1992), and FIG. 8 shows the cross section of a key section thereof. The CRT 21 is mounted liquid-tight via a first O-ring 52 fitted in an annular groove 51a provided at the periphery around an opening of a CRT frame 51 on the CRT 21 side thereof. And, the projection lens 28 is mounted liquid-tight via a second O-ring 54 fitted in an annular groove 53a provided at the periphery around an opening of a lens frame 53 on the lens side thereof.
Numeral 55 denotes a bellows which serves as a coupler, comprising a cylindrical bellows formed of an iron-based metallic material such as stainless steel which is energized in the axial direction, and is installed between the CRT frame 51 and the lens frame 53. The method of installation is as follows. The bellows 55 is provided with an annular member A 56 and an annular member B 57, each having a plurality of set holes 56a and 57a disposed at intervals along the periphery, which serve as mounting flanges and are welded on the respective ends of the bellows 55. The space within the bellows 55 is made liquid-tight by tightening screws 60 through the set holes 56a of the annular member A 56 via a third O-ring 58 fitted in an annular groove 51b provided at the periphery around an opening of the CRT frame 51 on the lens side thereof. Liquid-tight condition is obtained also by tightening the screws 60 through a set holes 57b of the annular member B 57 via a fourth O-ring 59 fitted in an annular groove 53b provided at the periphery around an opening of the lens frame 53 on the CRT side thereof.
Numeral 61 denotes a holding section which links a CRT block 81 whereon the CRT 21 is mounted on the CRT frame 51 via the first O-ring 52 and a lens block 82 whereon the projection lens 28 is mounted on the lens frame 53 via the second O-ring 54, forming an adjustment mechanism 98 which changes the relative angle and the distance between these members.
Now the operation will be described below. In the projector, which enlarges and projects the image generated by the CRT by means of the projection lens to form a large picture image, heat generated on the face 21c of the CRT 21 which reaches a very high temperature is transmitted via the coolant liquid 32 to the spacer block 22 made of aluminum of good thermal conductance and is dissipated therefrom. Therefore it is important to prevent the coolant liquid 32 from leaking, making it necessary to press the CRT 21 and the projection lens 28 against the mounting surfaces 22f and 22g of the spacer block 22 into tight contact, with the CRT packing 29 and the lens packing 30 being placed in the specified positions.
The coolant liquid 32, being made of a material such as ethylene glycol or silicon oil having a refractive index similar to that of glass, makes the light from the CRT 21 enter the projection lens 28 without refracting so that the deterioration of contrast is prevented, as well as providing the heat dissipation effect. Further, because the holding section 61 and the lens frame 53 shown in FIG. 7 are each provided with a spherical guide being formed thereon to allow relative rotary motion, the angle between the CRT 21 and the projection lens 28 can be changed by turning it. Adjustments of the relative angle and distance are carried out by means of the adjustment mechanism 98.
FIG.9 is a cross sectional drawing illustrative of the constitution of a projection type display apparatus disclosed in the Japanese Utility Model Application Laid-Open H2-143884 (1990). Numeral 203 in the drawing denotes a CRT mounting base having a substantially rectangular shape whereon a CRT 201 is mounted by means of an adhesive agent. Numeral 204 denotes a coupling frame having a substantially rectangular shape whereon a projection lens 202 is mounted with a packing 205 being interposed between thereof. The CRT 201 and the projection lens 202 are connected via the CRT mounting base 203 and the coupling frame 204 so that liquid-tightness is maintained. The CRT mounting base 203 and the coupling frame 204 are clamped with bolts 207 at four points with a packing 206 of substantially rectangular shape being interposed between thereof. A space 208 formed by the CRT 201, the projection lens 202, the CRT mounting base 203 and the coupling frame 204 is filled with a coolant liquid 209. Further, a part of the space 208 is made to serve as a bubble storage 208a to accommodate the change in the volume of the coolant liquid 209. In the conventional apparatus as described above, the relative angles between the CRT 201 and the projection lens 202 in the vertical and horizontal directions can be changed by adjusting the degree of tightening the bolts 207.
Projecting the image to focus on a point of the screen makes it necessary to set each projector unit at a proper convergence angle and correction angle. The conventional projection type display apparatus shown in FIG. 5 requires separate spacer blocks of three kinds to set proper values of convergence angle and correction angle for each projector unit, because the apparatus comprises the three projector units and has the CRT and the projection lens which are connected in a fixed structure. Moreover, production of a model of the apparatus having a different projection distance or a different screen size requires to manufacture a different spacer block which leads to longer design period and increased cost of dies. Furthermore, because the relative position of the CRT to the projection lens depends on the accuracy of machining the spacer block, high accuracy is required in machining the spacer block and it is difficult to correct the dimensional errors, once the apparatus has been assembled.
On the other hand, although the conventional apparatus disclosed in the Japanese Patent Application Laid-Open H4-352141 (1992) shown in FIG. 7 allows to change the relative angle between the CRT 21 and the projection lens 28, because the space 31 filled with the coolant liquid 32 is separated in the middle section by the bellows 55, four sealing members to keep the space 31 liquid-tight are required between the CRT 21 and the CRT block 81, between the projection lens 28 and the lens block 82, and between the CRT block 81, lens block 82 and bellows 55. This results in disadvantages of, in addition to the increase in cost, lower reliability such as higher likeliness of liquid leakage. Workability in assembly is also lowered because either the CRT frame 51 or the lens frame 53 interferes when fastening the bellows 55 with screws. And the bellows 55 made of a metal is poor in flexibility and has a disadvantage of being unable to change the inner volume even when the coolant liquid 32 expands due to heating of the CRT 21. Furthermore, installation of the bellows 55 requires longer distance between the CRT frame 51 and the lens frame 53.
The conventional apparatus shown in FIG. 9 has problems such as allowing to change the relative angle both in vertical and horizontal directions and the pressure exerted on the packing 206 becomes uneven from point to point when changing the relative angle, resulting in the possibility of deteriorated liquid-tightness.